The invention is directed to a power shovel including a dipper for gathering material from a bank and then moving the material to either a material pile or a truck for removing the material from the work site. More particularly, this invention is directed to a mechanism for reducing the likelihood of the dipper entering into what is known as a dipper stall condition. Still more particularly, this invention is directed to a mechanism for automating the behavior of the power shovel at dipper stall conditions.
A dipper stall occurs when the dipper enters a bank of material, and is then prevented from continuing any further, even though the hoist motor while operating at full power is attempting to raise the dipper. When this occurs, the motor power is highly reactive, and extra capacitors must be provided to absorb this reactive power. Dipper stall occurs when the resistance of the bank is greater than the force exerted by the hoist rope attached to the bail on the dipper. This excessive bank resistance occurs when the there is either too much material above the dipper, or when there is too much frictional force on the dipper.
In the prior art, an operator would visually observe the dipper movement through the bank of material. The movement of the dipper through the material was accomplished by operating the hoist motor at full speed to raise the dipper through the bank of the material, while at the same time moving the dipper handle forward at full speed to ensure the dipper enters into the bank of material. If the operator believed a dipper stall condition was about to be reached or had been reached, the operator would operate the crowd motor control to slow down or reverse direction of movement of the dipper handle. Once the dipper left the stall condition, the operator would again begin to move the dipper handle forward in order to move the dipper through the bank of material.
Some prior art power shovels monitored the hoist motor current and issued an alarm when the hoist motor current was reaching an upper limit, thus indicating to the operator that a stall condition has occurred.
This invention is a shovel including a dipper for gathering material from a bank, a platform, an upwardly extending boom connected at a lower end to the platform, and a sheave rotatably amounted on the upper end of the boom. The shovel further includes a winch drum mounted on the platform, a hoist motor for rotating the winch drum, and a hoist rope extending from the winch drum over the sheave and attached to the dipper. The shovel further includes a saddle block pivotally mounted on the boom, and a dipper handle slidably supported by the saddle block, and pivotable relative to the boom by the saddle block. The shovel further includes dipper handle moving means for moving the dipper and dipper handle away from and towards the boom, the moving means including a crowd motor and crowd motor operating means for operating the crowd motor, hoist motor operating means for operating the hoist motor, speed means for monitoring the hoist motor speed, and torque means for monitoring the hoist motor torque. The shovel further includes means communicating with the crowd motor operating means, the torque means and the speed means for moving the dipper handle toward the boom when the hoist motor speed is below a low-speed value while under high torque, and for permitting movement of the dipper away from the boom when the hoist motor speed is above the low-speed value or not under high torque.
In one embodiment, the dipper handle moving means further including means for varying the selected speed of movement of the dipper toward the boom, and the shovel further includes means for determining the amount of time the hoist motor speed is below a predetermined low-speed value while under high torque, and means for adjusting the selected speed of movement of the dipper toward the boom upward if the time the hoist motor speed is below a predetermined low-speed value while under high torque is too large, and for adjusting the selected speed of movement of the dipper toward the boom downward if the time the hoist motor speed is below a predetermined low-speed value while under high torque is too small.
In one embodiment, the shovel further includes means for determining the rate of change of the actual hoist motor speed while under high torque, and means for adjusting the low speed value upward if the rate of change of the actual hoist motor speed while under high torque is too fast, and for adjusting the low speed value downward if the rate of change of the actual hoist motor speed while under high torque is too slow.
In one embodiment, the speed means is a voltage sensor and the torque means is a current sensor.
One of the principle objects of this invention is to improve operator performance by improving the dipper fill factor. This is accomplished by ensuring the dipper is pulled out of the bank no further than necessary in order to prevent dipper stall.
Another of the principle objects of this invention is to increase the motor life of the hoist motor and to lessening the need for additional reactive power capacitance by reducing the amount of time the hoist motor operates at full load without actually hoisting the dipper.
Another of the principle objects of this invention is to lessen operator fatigue. In the prior art, the operator had to reverse the crowd motor every time a dipper stall occurred in order to pull the dipper out of the bank far enough so that there no longer is too much material above the dipper. After pulling the dipper out of the bank, the operator must then move the dipper handle forward again in order to once again cause the dipper to gather material. This invention permits the operator to keep the crowd motor control in it""s forward direction, thus eliminating the need to frequently reverse the crowd motor direction.
Another of the principle objects of this invention is to permit less experienced operators to successfully operate the shovel by no longer requiring the operator to know how to react to dipper stall conditions. It also permits operation of the shovel during the night. Without this invention, operators are not able to observe that the dipper has entered into a stalled state or to observe how far the dipper has been removed from the bank of materials. This invention also reduces operator training time, and improves the dipper fill factor by not permitting an operator to pull back too far on the dipper thereby reducing the amount of material which enters the dipper.